The invention relates to the problem of turbojet integrity following a fan failure.
Turbojets comprise an engine which drives a large diameter fan placed in front of the engine. The blades of the fan can become damaged following ingestion of a foreign body. In general, the fan is sufficiently robust to withstand the effects of ingesting such foreign bodies without too much damage, and it is capable of continuing to operate, perhaps at reduced efficiency.
Nevertheless, in some circumstances, the fan can be damaged to such an extent that it loses pieces of one or more blades. This gives rise to a large amount of unbalance which requires the engine to be turned off in order to reduce risks for the aircraft. Nevertheless, this unbalance created by losing blades gives rise to cyclical loading that is extremely large and that must be absorbed by the structure, at least while the engine is slowing down to the windmilling speed of the fan. The windmilling speed is the speed at which the engine rotates in its non-operating state as a result of the speed with which it is traveling through the atmosphere.
One usual way of eliminating the cyclical loading that needs to be absorbed by the structure consists in decoupling the rotary shaft of the fan from the support structure at the front bearing of the shaft. This is usually achieved by interposing fusible elements between the bearing and the bearing support structure, which elements break whenever the radial forces that the bearing needs to withstand exceed a predetermined value.
The fan shaft is then free to move radially to some extent and to turn about the longitudinal axis of symmetry of the engine, and the fan can start turning about an axis of rotation that passes substantially in the vicinity of its new center of gravity.
Nevertheless, under certain circumstances, the vibration that results from the unbalance which persists even at windmilling speed can still be very large. This is due to the natural frequency of vibration of the fan and to the loss of radial stiffness from the support bearing. Thus, in certain shaft support arrangements, means are provided for conserving a degree of stiffness for the bearing, or even for returning the axis of the shaft substantially onto the axis of the engine.
EP 0 874 137 provides for interposing a support element between the outer ring of the bearing and the support structure, which support element is normally held stationary by radially fusible elements and can slide in an annular cavity after the fusible elements have broken. The element then comes to bear against a damper which tends to return it towards its initial position. The annular cavity is in the form of a hollow cap defined by two concentric spherical walls against which the surfaces of the support element rub, thereby leading to inaccuracy in recentering.
U.S. Pat. No. 6,009,701 also describes a bearing for supporting a fan shaft in which the outer ring is secured to a stationary structure by radially fusible elements for the purpose of releasing the shaft relative to the stationary structure in the event of the fusible elements breaking. The outer ring is surrounded by a helically-shaped open ring which is capable of co-operating with a conical wall secured to the stationary structure. The conical wall presents a helical inner groove which enables the helical ring to move from one extreme position in which the clearance available to the shaft is at a maximum, towards the other extreme position in which the axis of the shaft again lies on the axis of the engine, as a result of the shaft rotating about the axis of the engine while the speed of the fan is slowing down from its operating speed to its windmilling speed.
The state of the art is also illustrated by U.S. Pat. No. 5,733,050 and U.S. Pat. No. 6,098,399.
In all those documents, it should be observed that the fusible elements are interposed between the outer ring of the bearing and the stationary structure of the engine. After the fusible elements have broken, the bearing is off-center relative to the axis of the engine. Unfortunately, the front bearing of the fan is fed with oil by nozzles secured to the stationary structure. Those nozzles can be damaged during the axial displacement of the bearing and this is mentioned expressly in U.S. Pat. No. 5,733,050. This can lead to the bearing being damaged during windmilling for lack of lubrication, should windmilling continue over a very long period.
The first object of the invention is to provide accurate recentering of the shaft after decoupling.
The second object of the invention is to conserve the integrity of the lubrication means after decoupling.
The invention thus relates to a device for radially supporting the front of a drive shaft for a fan of a turbojet of longitudinal axis X, the axis of said shaft normally coinciding with said longitudinal axis X, the device comprising a stationary annular support surrounding said shaft, a support bearing disposed between said shaft and said support and presenting an outer ring that is stationary in rotation and an inner ring that moves in rotation with said shaft, fusible means interposed radially between one of said rings and the adjacent part of the assembly constituted by said shaft and said support and designed to break on the appearance of a radial force of magnitude greater than a predetermined threshold so as to release said shaft radially relative to said support, and means for recentering the axis of the shaft on the longitudinal axis of the turbojet after said fusible means have broken.
The device is characterized by the fact that the radially fusible means comprise two sectorized annular webs or tenons provided in the respective radially outer regions of the inner ring and extending axially outwards therefrom, the ends of said webs or tenons being normally retained in the peripheries of two annular troughs that are axially spaced apart and constrained to rotate with the shaft, the radially inside face of the inner ring being radially spaced apart from the shaft so as to leave the shaft with clearance in the event of said webs or tenons breaking, and by the fact that the means for recentering the axis of the shaft on the longitudinal axis of said turbojet comprise two sets of balls each disposed between an axial face of the inner ring and the adjacent trough, each ball normally bearing against the bottoms of two facing indentations, one indentation in the adjacent trough and the other indentation in the adjacent front face of the inner ring, and resilient means urging said trough towards each other in order to return the balls towards the bottoms of said indentations.
Thus, in normal operation, when the radial loading applied to the bearing is of a magnitude below the predetermined threshold, the inner ring is secured to the troughs, and the balls are positioned in the bottoms of the indentations. When, following a failure of the fan, unbalance generates radial loading of a magnitude that is not less than the predetermined threshold, the webs or tenons of the inner ring break, and the shaft can move relative to the axis of the turbojet which coincides with the axis of the inner ring. The balls move up the slopes of the troughs and moves the troughs apart, thereby increasing the forces exerted by the resilient means until the rotor is rebalanced. Since the radial force decreases with decreasing speed of rotation, the shaft is recentered by the balls with very little friction force. During the period in which the troughs are off-center relative to the inner ring, the balls roll on the walls of the two indentations that face each other about the centers of the troughs, with friction force that is very low.
The device of the invention thus makes it possible to limit secondary damage to the engine and to the structure of the aircraft during the windmilling stage that follows decoupling. It should also be observed that the free radial displacement of the shaft is limited by contact between the shaft and the inner ring.
Most advantageously, each indentation is circularly symmetrical about an axis that is normally parallel to the longitudinal axis X.
Preferably, the resilient means bear against the axially outer face of one of the troughs, the other trough being axially stationary relative to the shaft.